1. Field of the Invention
The invention refers to vapor compression based mainly on vapor absorption. It applies to absorption heat pumps and heat to power conversion.
2. Description of the Related Art
The common way of heat transfer to higher temperature, as in cooling—refrigeration applications, is by using the vapor expansion—compression cycle. The refrigerant vaporizes at pressure p2 absorbing heat from its environment (a low temperature heat source), is compressed to a higher pressure p1, condensed rejecting heat at higher temperature T1 (high temperature heat source) and expands at the initial pressure p2. The refrigerant is a pure substance.
The vaporization and condensation temperature of a pure substance are the same. The higher the pressure the higher is the vaporization temperature. Vapor compression reflects the electric energy consumption of the cycle.
There is also an application where heat compression is applied. Now heat is consumed for working fluid compression. The solution is partially vaporized at high temperature. The vapor performs a cooling cycle while the solution expands and returns to absorber where the vapor is absorbed (condensed) at low temperature. The initial solution has been reformed, compressed and driven to evaporator. The application is called absorption or heat compression heat pump. The working fluid is a solution instead of a pure substance. The most common solutions are NH3—H2O and LiBr—H2O. The vapor pressure of a solution is a function of temperature and concentration as well. Ideal solutions follow the Raoult low P=xP0, where P0 the vaporization pressure of the pure substance at a given temperature and x the concentration. Real solutions divers from this low and P=αP0=γmP0, with α the activity, γ the activity coefficient being a function of concentration and m the mole concentration. It means that at a given temperature the solution may vaporize at different pressure depending on the concentration. The vapor is absorbed by the solution and thus condenses. In the followings the term vapor absorption or condensation is used. The compression ratio is DP=p1/p2=α1p01/α2p02=(α1/α2)(p01/p02). The ratio α1/α2 is unit in the usual absorption cycle as the solution has the same concentration during vaporization and condensation. Cooling coefficient of performance (cooling COP) of the absorption cycle is the evaporation heat of the pure solvent divided by that of the solution. The last is higher than the first so that usually COP=0.7.
There have been suggested methods where the solution concentration of the vapor generator or absorber exit is differentiated from the absorption solution concentration so that the second is higher than the first. Vapor is absorbed by the separated crystals. It results to higher absorption temperature than vapor generator temperature under the same pressure. In this way a limited temperature elevation may be achieved working basically in one pressure level.
In these cases, solution concentration differentiation is achieved by an external refrigeration circuit. This circuit cools the solution to a low temperature. The absorbent solubility lowers and absorbent crystals are formed and separated from the solution. It means that electric energy consumption is required. When the concentration differentiation is small the energy consumption is small and the temperature lift is small too. For high temperature lift, high concentration difference has to be created and the solution temperature has to be lowered very much. Thus the evaporator of the external (auxiliary) refrigeration cycle, will work at very low inlet pressure (corresponding to low refrigerant evaporation temperature). As the refrigerator inlet pressure lowers and required compression increases, energy consumption increases.
Besides, in case of using endothermic solutions, a high heat amount is consumed during vapor absorption by the crystals in the absorber, for crystals dissolution. It lowers the heating efficiency of the method.
Besides, in these methods, there is no solution expansion and compression or there is a small compression just to cover the pressure losses resulting from the evaporation and the cycle flows. There is no vapor compression either. Basically the cycle works under one pressure level (except of the external refrigeration cycle). Under these conditions, each built unit works only for a given external heat source and heat load (for a given low and high temperature sources). It means that they can not be used for space heating—cooling.
In other suggested methods, ion permeable membranes are used. The solution is compressed through a membrane so that almost pure refrigerant is separated. The refrigerant (low concentration solution) follows a refrigeration cycle and is absorbed by the rest of the solution (high concentration solution). In this case a solution of high concentration is required in order to create a considerable temperature lift. The required electric energy though, depends on the concentration and is very high. It may be higher than that of the known mechanical vapor compression cycle. Besides, membranes can not stand very high pressures.